A TFT that uses polycrystalline silicon (p-Si) can be used as a switching element of an active matrix display device (Patent Documents 1 to 3, for example). A TFT having polycrystalline silicon has a greater mobility than a TFT having amorphous silicon (a-Si), and therefore, it is possible to form not only pixel TFTs for conducting display, but also driver circuit TFTs, which require fast operations. For the pixel TFT, a TFT with a small off-current is needed in order to maintain a high display quality, and for the driver circuit TFT, a TFT with a large on-current is needed because of the need for fast operations.
A structure of a TFT in which a lightly doped drain region (sometimes referred to as LDD region below) is formed on at least one of the space between a channel region and a source region, and the space between the channel region and a drain region is widely used (Patent Documents 1 to 3, for example). Such a configuration is referred to as an “LDD structure.” By forming the LDD region, the electrical field concentration in the vicinity of the source region or the drain region can be lowered, thus allowing the off-current to be small and improving the long term reliability.
Patent Document 1 discloses a semiconductor device that has an LDD region in both the driver circuit TFT and the pixel TFT, in which the impurity concentration in the LDD region of the n-type pixel TFT is lower than the impurity concentration in the LDD region of the n-type driver circuit TFT, for example. With this configuration, the off-current of the pixel TFT can be made small without reducing the on-current of the driver TFT. As a result, a liquid crystal display device in which no display unevenness occurs, and in which the operation speed of the driver circuit is fast can be attained.
Patent Document 2 discloses a semiconductor device that has an LDD region in both the driver circuit TFT and the pixel TFT, in which the LDD region of the n-type pixel TFT is longer than the LDD region of the n-type driver circuit TFT. With this configuration, the off-current of the pixel TFT can be made small without reducing the on-current of the driver circuit TFT. As a result, a liquid crystal display device in which no display unevenness occurs, and in which the operation speed of the driver circuit is fast can be attained.